You can edit your posts, at least for some time. So you can edit it, and replace the text with 'deleted'. You might even bet by with a blank or '.' You will see an 'edit' button at the bottom of the post.
Just tested and blank won't do but . will (see post above). But as Nealh has pointed out it is generally bad practice as it makes replies meaningless. No problem if done quickly (eg hit send by mistake) as there won't be any replies. Useful for error correction, clarification or adding detail.